During the establishment of an oilfield operation, several phases of production can be distinguished:                First, the production of oil due to the internal pressure of the field, which is usually quickly reduced;        Then, pressure is maintained by injecting water (water flooding) until the producing wells reach an oil/water concentration of 2 to 5%. At that time, the cost of water injection makes the cost of the oil noncompetitive;        Next, enhanced oil recovery by various methods.        
One of the methods consists in injecting a viscous solution of polymers, particularly polyacrylamide, in order to mobilize a larger volume of the field.
This method was industrialized during the first oil crisis in 1973.
However, the injected polyacrylamides are sensitive to the mechanical and chemical deterioration of the molecular weights, which requires precise handling in order to maintain their effectiveness.
In general, polymers are in the form of powder that is dissolved in water at a rather high concentration, in order to reduce the size of the maturation vessels, i.e. dissolution vessels. Indeed, the dissolution time of these powders is on the order of one hour. The concentration of these solutions is from 10 to 20 grams/liter with high viscosities (1000 to 10,000 cps).
To be used, this stock solution must be diluted to 1 to 3 grams/liter in order to be injected. Various methods are used to prevent mechanical deterioration by a sharp pressure drop, in order to adapt the injection pressure to the pressure of the specific well.
The first consists in installing one metering pump per well, and sending the stock solution to the head of each well in order to dilute it with the injection water.
The second method consists in diluting the stock solution on the dissolution platform, and of sending this diluted solution into each well by separate pipelines.
The third method consists in totally diluting the stock solution with all the injection water, and of using water injection lines. This method requires the lowest investments.
In this case, the water pump will be set higher than the pressure of the well that has the highest pressure, and the pressure on the other wells will have to be reduced; this normally results in a deterioration that increases as the pressure differential increases.
To prevent this deterioration, specific pressure reducers are used that should not deteriorate the polymer.
Several methods are possible:                Pressure reduction by a long, calibrated tube that allows pressure reductions of up to 60 to 100 bars, with less than 5 or 10% deterioration depending on speed and the length of the tube (document US 2012/0292029).        
The optimal speed is on the order of 10 meters/second. Normally, a pressure drop of 1 bar per 10 meters of tube can be created, or 500 meters for 50 bars. According to this patent, the pressure drop can be modulated by cutting the tube into various lengths that can be fed or bypassed.                Pressure reduction by vortex effect with a tube equipped with caps creating a vortex effect.        
U.S. Pat. No. 4,782,847 utilizes fixed caps producing a pressure loss without deterioration up to pressure differences of 5 bars, and a modulation needle valve that also works by vortex effect and which allows a variation from 0 to 5 bars depending on the opening, with a minimum of deterioration.                Pressure reduction by vortex effect with a plurality of needle (or other) valves that can each create a pressure drop of about 5 bars with minor deterioration (document US 2016/0168954).        
With about 10 valves, a pressure drop of 50 bars can be created. The valve heads can be equipped with hydraulic motors in series, enabling the same opening on all valves with a single adjustment.
These systems are cumbersome for the first one, not very scalable for the second one, and rather expensive for the third one.
The demand of oil companies is for maximum pressure drops of about 50 bars, with little deterioration, with a modulation per well of about 10 bars, and a possibility of feeding wells whose pressure varies over time, with a simple, inexpensive apparatus.
The equipment must therefore be very flexible and easy to handle.